Jack lands the plane and Mac sees that it has a hole in the petrol tank, which is leaking. He then gets a stick and shoves it into the hole, stopping the leak. But the puddle of fuel underneath is way too big. Considering the size of the hole, and the time it took Mac to plug the leak, it couldn't have leaked so quickly.
Me thinks they just chucked a bucket of water on the ground and pretended that was fuel.